I'm in the middle of upgrading a Rails app from 2.3 to 3.2. There are many places where html strings are constructed and rendered. This monkey-patch has been a big help in finding all the places I need to use raw or html_safe.

warn_on_escaped_html.rb

module ActionView
  class Template
    def render_with_warn_on_escaped_html(*args,&block)
      output = render_without_warn_on_escaped_html(*args,&block)
      unless Rails.env.production? || args[0].request.instance_variables.include?(:@skip_escaped_html_check) 
        if output =~ /^.*<.*$/
          warn "WARNING: Escaped HTML in #{ Rails.backtrace_cleaner.clean(caller).first }: #{Regexp.last_match}"
          args[0].request.instance_variable_set(:@skip_escaped_html_check,true)
        end
      end
      output
    end
    alias_method_chain :render, :warn_on_escaped_html
  end
end